Obesity (body mass index, BMI >30) afflicts millions of people in the United States and other countries, and is a major risk factor for heart disease, type II diabetes mellitus, stroke, hypertension, and morbidity. The G-protein coupled melanocortin-3 receptor (MC3R) is expressed in the central nervous system (brain) and is part of the melanocortin pathway involved in the regulation of energy homeostasis. The specific role of the MC3R in the regulation of obesity has not been clearly defined due to a lack of receptor specific ligands and a complex metabolic phenotype of the MC3R knockout mouse. This project is focused upon the drug discovery of MC3R selective molecules (peptide and small molecules), in vitro lead candidate selection, and use of wild type and knockout mice for further molecule lead selection and to probe the role of the MC3R in the novel hypothesis of the MC3R directly involved in the regulation of food intake and satiety. It is anticipated that MC3R ligands have the potential to become therapeutic ligands for obesity related diseases that bypass the human melanocortin-4 receptor (MC4R) agonist associated side effects of male erectile activity and hypertension.